The present invention relates to apparatus for effecting the fine-adjustment of the position of a scraper blade and particularly of a lip of the head-box of a paper-making or cardboard-making machine. It also relates to a method of adjusting a property of material produced on a continuous basis on a machine having a scraper blade, for example a machine for making paper or cardboard, by effecting fine adjustment of the position of the scraper blade.
The description that follows relates to the fine adjustment of the position of the lip of a head-box or starting box of a paper-making or cardboard-making machine, but it should be pointed out that it has other applications in fields in which the adjustment of a scraper blade is called for.
The head-box of a paper-making machine converts the flow of fibrous suspension, forming the paper slurry, from a cylindrical stream into a layer corresponding to the width of the sheet to be formed. This head-box takes the form of a reservoir of variable shape, the front face of which has, towards the bottom, a slot provided with lips, between which the liquid slurry is projected onto a production wire. The purpose of the head-box is to ensure a constant delivery and to adjust the fibrous suspension over the entire width of the machine, which may be as much as nine meters in the case of modern installations.
The layer containing fibres in suspension as well as mineral filler is forced under a pressure of between approximately 0.03 and 2 bars between the lips in modern high-speed machines. The evenness of the rate of discharge of the layer containing fibres in suspension and the uniformity in thickness, together with the uniformity of the concentration determine the uniformity of the weight per unit area of the manufactured product. In modern paper-making machines, flow takes place between two metallic lips, one of which is fixed whereas the other is movable as a whole so that it adjusts the thickness. Furthermore, the movable lip, which is usually the upper lip, is deformable lengthwise under the action of rods controlled by manually-operated screw-jacks. A modern head-box comprises several dozens of such manually-operated screw-jacks.
The correction of the thickness of the layer of material containing fibres in suspension as it passes the lip, as a function of the changes in weight per unit of area of the product obtained at the end of the machine, is not very conveniently carried out with the aid of such manually-operated screw-jacks, the adjustment of which is a delicate matter and which gives good results only on a trail-and-error basis.
To eliminate this disadvantage, head-boxes have been designed wherein the manually-operated regulating screw-jacks are each controlled by a motor-reducer unit which is itself controlled in dependence upon continuous measurements of mass, by way of a computer which receives the results of the measurements of mass and calculates the corrections to be made by acting on the machine elements and particularly on the movable lip.
These control systems using motor-reducers are not sufficiently satisfactory. In fact, relatively great mechanical back-lash always occurs, and precision in adjustment is not very great. However, the greatest drawback is the very high cost of these mechanisms, since the head-box of a modern machine may comprise up to sixty or more of them.
U.S. Pat. No. 2,779,253 describes a purely mechanical means for adjusting the movable lip of a head-box of a paper-making machine. According to that Patent, adjustment is carried out on a purely mechanical basis with the aid of screw-jacks, and variations are detected by means of a mechanical comparator. French Pat. No. 1,192,516 describes apparatus for adjusting the orifice through which the paper slurry passes from a head-box, the adjustment being achieved by inflating and deflating rubber-bellows devices which form the edges of the lips. Thus, the device is pneumatically or hydraulically operated.
On the other hand, U.S. Pat. Nos. 2,938,231 and 3,940,221 describe dies for the extrusion of plastics material wherein the position of part of a first side of the die is adjusted with the aid of a heat-expansible device; in particular, the above-mentioned U.S. Pat. No. 3,940,221 describes a guide for the extrusion of plastics material that comprises a block, one part of which, designed to delimit the outlet orifice, is separated from the body of the block by a part of reduced thickness so that it acquires a certain resilience. A heat-expansible rod applies varying degrees of thrust to this flexible end so as to determine its position in a precise manner.
Dies for extruding plastics material are devices that are totally different from the lips of the head-boxes of paper-making machines. In fact, the plastics material passing through an extursion die is moved under a very high pressure. The die must comprise, at least over a certain distance, a duct having substantially parallel walls so that they continually guide the material which, in effect, is aligned when it flows into the die orifice. In contrast to this, the lip of a head-box of a paper-machine is formed by a scraper blade which is set, relative to the surface of the formed layer, at a relatively large angle, generally at least 30.degree. and sometimes as much as 90.degree.. A lip of this kind must not, in any event, cause orientation of the fibres suspended in the material that passes below it. In fact, such orientation of the fibres would be disastrous in the finished paper (producing a direction of preferential tearing). It is therefore essential not only that the lip be considerably inclined in relation to the formed layer of product material, but also that the pressure on the material containing fibres in suspension be slight.
It is therefore obvious that the lip of a head-box of a paper-making machine is a device that is totally different from a die for extruding plastics material.
In view of the fact that there are at present in existence means for continuously measuring mass and that these means can be connected to computers which are able to recognise the need for corrections and to calculate their amounts, it is very desirable to provide simple and inexpensive devices for continuously effecting fine adjustment of the position of the moving lip of a head-box of a paper-making machine at different points along this lip.